DESCRIPTION: The innate and adaptive immune response involves the recruitment of leukocytes from the blood stream to the site of infection and inflammation. Upon reaching the location, leukocytes clear invaders and begin the process of digesting and repairing damaged tissues. However, when the body fails to properly regulate the recruitment of leukocytes, the inflammation can become chronic, resulting in irreversible tissue injury and loss of functionality. Rheumatoid arthritis, inflammatory bowel disease, type-1 diabetes, and multiple sclerosis are all examples of autoimmune diseases caused by the uncontrolled recruitment of leukocytes. While much research has been dedicated to the identification of the cascade of specific biochemical processes involved in the recruitment of leukocytes, much less is known about the mechanical events driving their migration, in particular how they generate the necessary traction forces to cross the vascular wall and further traverse the three- dimensional (3-D) extravascular space. Thus, the main objective of this study is to provide the much needed complementary information connecting specific cell molecular processes (i.e., adhesion dynamics, actin turnover, and myosin II contraction) to the generation of cellular forces that regulate leukocyte extravasation and their subsequent directional migration in 3-D extravascular tissues through the use of novel 3D Fourier Traction Force Microscopy (3DFTFM) techniques and genetic and pharmacological manipulations. To achieve this objective, we propose three Specific Aims. We will first characterize the temporal and spatial generation of 3-D traction forces exerted by leukocytes crawling on flat surfaces (Aim 1); we will then investigate the mechanical processes regulating transmigration across the vascular endothelial monolayer and the basement membrane (Aim 2); and finally, we will develop a novel Elastographic 3DTFM to determine both traction stresses and the non-linear material properties of the Extra Cellular Matrix to elucidate the molecular mechanisms regulating the mechanics of leukocytes' chemotactic migration in 3-D environments (Aim 3). The proposed in vitro approach overcomes a number of existing challenges to measuring the 3-D traction forces driving leukocyte extravasation and migration and builds on the extensive experience accumulated by our multidisciplinary team of biologists and engineers who have been studying the mechanics of amoeboid cell migration for the last seven years. The outcome of this research will result in a far more comprehensive understanding of the mechanics of leukocyte motility than that available to date and will have the potential to aid the development of new approaches that could target specific mechanical processes to inhibit (or slow down) leukocyte motility and help in the design of complementary regimens to treat inflammatory diseases.